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United States Patent |
5,235,861
|
Seppa
|
August 17, 1993
|
Power transmission line monitoring system
Abstract
A system for determining the sag of an overhead power transmission line to
prevent flashover to adjacent objects on earth. A tension measuring device
is provided to produce a sign as a function of powerline tension. The
signal is processed and transmitted to a distant location by cellular
communication to a distant central monitoring station. Knowing the
sag-tension relationship of the monitored line the sag is determined. Thus
action may be taken if the powerline sag becomes excessive.
Inventors:
|
Seppa; Tapani O. (39 N. Valley Rd., Ridgefield, CT 06877)
|
Appl. No.:
|
695096 |
Filed:
|
May 3, 1991 |
Current U.S. Class: |
73/862.391; 73/862.541 |
Intern'l Class: |
G01L 005/04 |
Field of Search: |
73/862.39
307/62
340/870.3
174/40 R,40 TD,45 R,45 TD,149 R
364/492,508,550,556
|
References Cited
U.S. Patent Documents
3073156 | Jan., 1963 | Rowe | 73/862.
|
3098988 | Jul., 1963 | Hafner | 174/40.
|
3759094 | Sep., 1973 | Al | 73/862.
|
4402229 | Sep., 1983 | Byrne | 73/862.
|
4409429 | Oct., 1983 | Gaylard | 174/40.
|
4786862 | Nov., 1988 | Sieron | 324/127.
|
4837800 | Jun., 1989 | Freeburg et al. | 455/33.
|
Foreign Patent Documents |
0011624 | Jan., 1986 | JP | 73/862.
|
0754541 | Aug., 1980 | SU | 174/40.
|
0974483 | Nov., 1982 | SU | 174/40.
|
Primary Examiner: Chilcot, Jr.; Richard E.
Assistant Examiner: Dougherty; Elizabeth L.
Attorney, Agent or Firm: Hyde; Edward R.
Claims
What is claimed is:
1. A system for measuring the sag of an overhead power transmission power
line comprising:
a power transmission line section having two terminal ends;
two section terminal transmission towers;
strain insulator means to secure each terminal end of the power line
section to a respective section transmission tower;
intermediate transmission towers spaced in a line between the terminal
towers;
suspension insulator means to secure intermediate points of the power
transmission line section to respective intermediate transmission towers;
said suspension insulator means to permit longitudinal movement of the
power line;
tension measuring means interposed between each strain insulator means and
the respective terminal transmission tower to produce an electrical signal
representative of the power line tension;
signal processing means located at and connected to the tension measuring
means;
said signal processing means including programmed computer means to produce
second electrical signals which are a function of the sag of the power
line; and
transceiver means connected to the computer means to transmit the second
electrical signals to a distant location whereby current in the power line
is adjusted in accordance with the received second electrical signals.
2. The system set forth in claim 1, including means at the distant location
to transmit control signals to the said transceiver means to control the
signal processing means.
3. A method for monitoring the sag of an overhead power transmission line
comprising the steps of:
measuring the tension of the power line;
producing an electrical signal representative of the tension measurement;
processing said electrical signal in accordance with a predetermined
tension-sag relationship to produce a second signal which is a function of
sag of the power line;
transmitting said second electrical signal to a distant location in a
predetermined transmission mode;
receiving the second signal at the distant location whereby current in the
power line is adjusted in accordance with the received second signal.
4. The method set forth in claim 3 in which the transmission mode is a
continual transmission of the processed signal.
5. The method set forth in claim 3 in which the transmission mode i san
intermittent transmission of the processed signal.
6. The method set forth in claim 3 in which the transmission mode is a
transmission of the processed signal of a predetermined threshold.
7. The method set forth in claim 3 in which the data transmission method is
a cellular radio telephone.
8. The method set forth in claim 3 in which the step of measuring the
tension of the power line includes the steps of measuring the inclination
angle of the power line and calculating the horizontal load of the power
line.
9. The method set forth in claim 3 in which the second electrical signal is
a function of power line sag in accordance with the formula
##EQU2##
which D=sag.
m=conductor weight/unit length
S=span length and
H=horizontal tension.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system used for monitoring the transmission
capability of electrical power transmission lines. The transmission
capability of power lines will vary with the temperature of the line
because the current carrying capacity is limited by the allowable sag of
the line between transmission towers. In order to adequately control the
load on a line it is necessary to know the condition of the line in terms
of the sags of its spans. The present invention provides a system for
monitoring such overhead transmission lines so that the load on the line
may be adequately and properly controlled.
2. Description of the Prior Art
A major problem in overhead power transmission lines is that of clearance
between the line and the nearest point on earth. If the line becomes too
close to the earth or adjacent structures, problems can arise from
electric flashover from the power line which can cause extensive
electrical damage. For this and other reasons it is necessary to limit the
current of these transmission lines so that any undue sag may be
prevented. The major cause of increased sag is that of heating of the
line. As the temperature increases the power line expands and clearance
problems may arise.
The heating of the line results from various sources. One cause is the heat
that is generated in the conductor by the electrical current flowing
through it which causes I.sup.2 R losses in the conductor. Thus as the
current in the line increases there is a greater generation of heat with
the resulting increase in line sag. Line heating is also effected by solar
heat and ambient temperature in the surrounding area. This heating of the
line is offset by the cooling effect of wind passing over the line and
heat radiated from the conductor.
Traditionally, transmission lines were rated based on an assumed
combination of worst cooling conditions, consisting of a combination of
expected highest ambient temperature, solar radiation and a low wind
speed. Such traditional current ratings were highly conservative. To take
adavantage of this conservatism, methods have been developed to either
monitor some of the cooling conditions or the actual temperature of the
conductor and to adjust current ratings based on such monitored data.
In the prior art, a number of methods have been used for determining line
temperature. A first such method is that of theoretical calculation.
Assumptions are made of wind speed and direction, ambient temperature and
solar radiation and calculations are made for arriving at the line
condition. Because the calculations are based on theoretical assumptions,
the result can be at considerable variance from the actual line condition
which might permit greater line current than exists or on the other hand
dictate a lower actual line current.
In some instances weather stations have been established in the general
location of the transmission line in order to monitor the weather to thus
provide somewhat more reliable data that is then used to calculate the
line conditions including the temperature of the line. A third method for
monitoring the line known to the prior art is to provide sensor devices
mounted on the conductor along the length of the line at various intervals
to measure conductor temperatures from which load capacity can be
determined. These various systems of the prior art are disclosed for
example in U.S. Pat. Nos. 4,268,818 and 4,420,752 and 4,806,855. These
later monitors have been somewhat more effective in identifying actual
temperatures of the conductors. However one drawback of such systems has
been that such sensors provide a measurement only at one point on the line
and large number of sensors are thus required to cover a long span or
series of spans because temperatures will vary considerably along the
length of a span. Further they require extensive special communications
methods such as FCC site specific radio licenses. Furthermore, because the
sensor modules are mounted on the energized conductor, the manufacturing
and installation cost of the sensors is complicated and expensive.
A further disadvantage of the prior, conductor temperature based, rating
methods is that they cannot take into account creep, which is progressive
stretching of conductor, caused by variation of conductor loading. The
design sag and tension tables of conductors, such as the one shown in
Table 1, determine the conductor tension and sag in initial condition
(before any creep) and final condition (after calculated maximum creep).
As shown in Table 1, the resulting uncertainty between the sags can be
more than 10% of sag, and equivalent to a temperature uncertainty of 25 to
30 degrees C. This uncertainty is eliminateed if the lines are rated based
on conductor tension.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the prior art by
monitoring line tension which remains substantially constant over several
spans of the transmission line. The measurements are recorded in a module
located at a transmission tower and processed to determine that the lowest
design tension, corresponding to the maximum allowable sag, is not
exceeded. Because the tension of the line varies relatively slowly with a
typical time constant of 10 to 15 minutes, the tension measurement may be
made intermittently with the interval between measurements depending upon
the size of the line conductor. The collected data is transmitted to a
distantly located central processing station.
The communications can be based on any available common carrier or
dedicated communications system. Nevertheless, because of the remote
location of the measurement systems and the infrequent need for
communication, the invention envisions use of cellular telephone
communications as the preferred communications method. Likewise, because
the power demand of the system is very low it can be powered by many
commercial sources. In remote locations it is envisioned that the most
economical power source is likely to be solar cells which feed a backup
battery.
It is therefore a primary object of the present invention to provide a
system for monitoring the condition of an overhead transmission powerline
to provide efficient operation thereof.
It is further object of the present invention to provide a transmission
powerline monitoring system in which sensors are provided at intervals
along the line that measure the tension of either individual line spans,
or multi-span sections of the line.
Another object of the present invention is to provide a method for
measuring the tension of spans of an overhead transmission powerline and
efficiently transmit the measured information to a distant central
processing station.
It is still further object of the present invention to provide a system for
measuring and monitoring the tension of spans of an overhead transmission
line and utilize the measurements to calculate the optimum load carrying
capacity of the transmission line.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and still other objects and advantages of the present
invention will be more apparent from the following detailed explanation of
the preferred embodiments of the invention in connection with the
accompanying drawings herein in which:
FIG. 1 is a view of a transmission tower having electric power transmission
lines showing schematically the system of the present invention;
FIG. 2 is a portion of a transmission line suspension system;
FIG. 3 is a general block diagram of the components of a sensor module; and
FIG. 4 is a schematic view of the sensor and associated module.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An overhead transmission powerline comprises a series of transmission
towers having spans of transmission line swung between them. The portion
of the transmission line between adjacent towers is customarily designated
a span and a series of such spans make up a suspension section. Each end
of a suspension section is secured to a tower by a strain insulator
structure which is subject to the full tension of the line connected to
it. The individual suspension spans on the other hand, terminate at their
respective towers by suspension insulator strings which permit significant
longitudinal movement of the ends of the suspension spans.
Referring now to FIGS. 1 and 2, 10 indicates a transmission tower that
terminates a suspension section of a power transmission line generally
indicated by 12. The section is made up of a number of suspension spans
12A, 12B to 12G which terminates at its tower 14. The terminal ends of the
section are secured to their respective towers 10 and 14 through strain
insulator strings, also known as dead end strings 16 and 16A. The
intermediate suspension spans are secured to their respective towers by
suspension strings in such a manner that the ends of the spans may move
laterally and longitudinally.
For example, span 12B is secured at its ends to suspension strings 18 and
18A which are suspended in a vertical manner to their respective towers
represented diagrammatically, as 13, 15. Thus the strain structures 16,
16A are subject to the full tension of the line. Because the suspension
strings 18, 18A hang from their respective towers permitting longitudinal
movement of the end of the sections, the horizontal tension equalizes and
is substantially constant throughout the suspension section.
One end of each suspension section has a tension measuring device
interposed between the strain insulator and the associated tower.
Thus the end of the section of powerline 12 joins to tower 10 through the
strain insulator 16 and the tension measuring device 22. The latter may be
a load cell of the strain gauge type and it has been found that a
commercially available load cell such as the Allegheny 301 is very
satisfactory for this purpose. Although the line 12 secures to tower 10 it
does not terminate but rather continues on to the next adjacent tower as
shown by 21 in FIG. 1.
As the temperature varies, the powerline 12 will expand or contract with a
corresponding change in tension. The tension will substantially equalize
throughout the length of the section because of the suspension string
sections 18, 18A, etc. and this tension will be measured by load cell 22.
The latter is connected to a console 24 by a shielded cable 26 and is
secured to any convenient position on the transmission tower 10.
The console 24 will contain the various electrical components shown in FIG.
3. The analog signal from the load cell 22 is passed to an amplifier and
then to bridge amplifier and A/D converter 23 which converts it to digital
form. This digital signal is then connected to a CPU 25 where it is
appropriately processed as will be hereinafter described and passed
through to a cellular transceiver 27. The various components of the module
are powered by a solar power supply 28. The latter is shown in FIG. 4 as a
solar array 30 held up by a mast 32 which also supports the cellular
transceiver antenna 34.
Thus it is seen that the tension of the section is monitored by the system
and the results of the tension measurement may be conveyed through the
cellular transmission link to a central station where the tension is
converted to line sag measurement. It is understood that for a particular
type of powerline conductor and span length the relationship between
tension and sag is known. For transmission line spans of moderate length
and on level ground, one can use with high accuracy the simple parabolic
formula:
##EQU1##
in which: D=sag, m=conductor weight/unit length s=span length and
H=horizontal tension.
Thus the sag of a given span is inversely proprotional to the horizontal
component of the tension. The required simple computation for the sags of
each span of the suspension section can be done either at the modules
located at transmission towers or at the central processing station.
Slightly more complex, hyperbolic, equations for sag computation are used
for exact determination of sags for inclined or long spans. Standardized
software packages are commercially available for such calculations.
Conductor manufacturers generally provide such data. For example, the
Aluminum Company of America provides a sag tension table for a 1000 ft.
span of conductor designated ACSR Drake, 795.0 KCMIL, 26/7 stranding for
which the stress-strain data is as follows:
______________________________________
FINAL INITIAL
SAG TENSION SAG TENSION
FT LB FT LB
______________________________________
24.99 12592. 24.97 12600.*
18.79 7292. 16.99 8060.
20.84 6578. 18.64 7350.
22.86 5999. 20.35 6735.
24.82 5527. 22.08 6209.
26.72 5137. 23.81 5760.
28.79 4771. 25.75 5329.
______________________________________
For the actual design of the suspension sections of the line, the design
engineer uses sag/tension tables as the one shown above and has designed
each of the spans of the suspension section in such a manner that the sag
limitations will not be violated if the longitudinal component of tension
remains higher than the particular tension which he concluded is the low
limit of the longitudinal component of tension.
Thus, with knowledge of the tension, the operator at the distant location
can appropriately adjust the electrical current of the line to insure that
the critical sags for the particular line are not exceeded. Use of tension
as the primary determining factor for thermal rating eliminated the
uncertainty of the conductor creep. It also allows accurate rating a
complete suspension section of the line with single monitoring device.
The system may operate in various modes and as described above, there may
be a continuing monitoring of the tension which is passed on to a central
station where the sags can be determined and the system appropriately
managed. An alternative arrangement would be to sample the tension at
predetermined intervals under the control of a clock in the CPU at the
tower module. The sampling might, for example, be made every five or ten
minutes. These intermittent results of the line tension would be stored in
the system and transmitted to the central station on demand.
Another mode would be to have a critical alarm tension programmed into the
computer and when the line tension approaches the critical tension an
alarm would be transmitted to the central station. The alarm conditions
could be selectively programmed to the computer software depending on the
particular user's practices. Changes in operation of the system can be
made using the cellular communication from the ground station to the tower
module.
An additional feature of the system is that the same electronic console 24
may also process the tension of the next succeeding suspension section. As
seen in FIG. 1, a load cell 31 and strain insulator 33 secures the end 35
of the next section to tower 10. The load cell connects by a shielded
cable (not shown) similar to cable 26 to the console. Thus two adjacent
sections may be conveniently monitored and the results transmitted to the
distant central station by the same set of computer and communication
equipment.
The invention has been described in which tension is measured directly by a
load cell. Alternatively, tension may be determined indirectly with an
inclinometer at the dead end strings in a similar location as the load
cell shown in the drawings. The inclinometer would measure the incline
angle of the line. Because the verticle component of the load remains
constant and equal to the weight of the span, the horizontal load can be
resolved from the equation
H=W.alpha.
where
W=known verticle load and
.alpha.=inclination angle.
The inclinometer may be any of a number that are commercially available.
Having thus described the invention with particular reference to the
preferred forms thereof, it will be obvious that various changes and
modifications may be made therein without departing from the spirit and
scope of the invention as defined in the appended claims.
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